The present invention relates to a system for driving a DC motor, more particularly, to a system for driving a DC motor employed in, for example, a large magnetic disc system for driving a spindle therein, and connected to a constant current drive circuit.
FIG. 1 is a diagram of a prior art bi-polar direct current (DC) motor drive system employing a very bulky magnetic disc system (not shown). In FIG. 1, reference 1 denotes a three-phase brushless Hall-type DC motor including three exciting coils 2A, 2B, and 2C, and Hall-effect-type sensors 3A, 3B, and 3C, 4 denotes a circuit for synthesizing outputs SHG.sub.A to SHG.sub.C from the Hall sensors 3A, 3B and 3C, 5 denotes a timing control circuit, 6 denotes a phase exchange switching circuit, 7 denotes a power supply (power source), and 8 denotes a constant current drive circuit having a phase switching function.
The phase exchange switching circuit 6 includes three power transistor type switches 6A to 6C. The constant current drive circuit 8 also includes three constant current sources 8A to 8C each having at least one power transistor.
A rotor (not shown) of the DC motor 1 is mechanically connected to a spindle (not shown) of the magnetic disc system, rotating a magnetic disc)s) (not shown) in response to the rotation of the spindle.
The rotation position of the rotor of the DC motor 1 is detected by the Hall sensors 3A to 3C. The signals SHG.sub.A to SHG.sub.C output from the Hall sensors 3A to 3C are synthesized at the signal synthesizing circuit 4, resulting in a phase signal SPHASE. The timing control circuit 5 generates timing signals ST.sub.A to ST.sub.C for energizing the power transistor switches 6A to 6C and control signals SC.sub.A to SC.sub.C for controlling the constant current sources 8A to 8C, in response to the phase signal SPHASE. As a result, series-connected exciting coils 2A and 2B, 2B and 2C, and 2C and 2A are consecutively energized in response to the phase signal SPHASE, to rotate the rotor of the DC motor 1.
Generally, the motor has a predetermined relationship between the drive power and torque (or mechanical energy). Accordingly, by controlling the drive current, the torque generated in the motor can be freely controlled. In other words, when a load on the motor is varied, the torque generated in the motor can be maintained at a predetermined constant value by supplying a constant current to the exciting coils. In addition, in the DC motor, a large start-up current may flow into the coils for a lengthy start-up time, due to a large inertia of the rotor. This basically requires a bulky and high-cost power supply for supplying sufficient start-up current during a long start-up time. When the constant current drive circuit is provided, the start-up current is very limited, enabling a reduction of the power supply. As discussed above, the constant current drive circuit 8 contributes to obtaining the above advantages. Furthermore, when the constant current drive circuit is employed for a phase-exchange-type DC motor as shown in FIG. 1, and accordingly, may include switching power transistors, the constant current drive circuit provides the phase exchange function.
Referring back to FIG. 1, in the DC motor 1, a counter-electromotive-force (emf) is induced in the exciting coils 2A to 2C during the rotation of the rotor, and the amplitude of each counter-emf is enlarged in response to an increase in that rotation. Accordingly, a voltage of the power supply 7 is designed so that it will overcome the counter-emf at a required high rate of speed, e.g., 3600 RPM, of the rotor and enable a constant current control.
The characteristics of the DC motor can be expressed by the following formula: ##EQU1##
where, V.sub.M : voltage supplied to the motor (V),
K.sub.e : induced voltage constant (V), PA0 R.sub.S : speed of the rotor (RPM), PA0 L: inductance of the series-connected coils (H), PA0 r.sub.M : resistance of the series-connected coils (.OMEGA.) and PA0 i: current flowing through the series-connected coils.
During the start-up operation of the motor, or at a low speed operation, the speed R.sub.S is almost zero or very low and the counter-emf is almost zero or very small. As a result, in spite of the provision of the constant current drive circuit 8, a large current is still supplied to the power transistor type switches 6A to 6C and the power transistors in the constant current sources 8A to 8C, and accordingly, these power transistors accumulate heat. The start-up time may be approximately 25 to 35 seconds when the DC motor is used for driving a large-scale magnetic disc system. Therefore, taking these conditions into consideration, high power transistors having a tolerance for a large current passing therethrough and a high temperature thereat during a lengthy start-up time must be provided. This results in the disadvantages of high cost, a bulky circuit configuration, and the installation of expensive and bulky cooling members. In addition, the probability of breakage of the power transistors is increased, reducing the reliability of the DC motor drive system. Among other elements, the power transistors of the switches 8A to 8C suffer from the latter problem, because these transistors are used in a linear region of the characteristics thereof.
A strong demand for a reduction or elimination of the above problems has arisen.
JPA No. 57-183281, published on Nov. 11, 1982, discloses a speed control circuit for a brushless DC motor. As shown in FIG. 4 of JPA No. 57-183281, the circuit avoids the application of excess power to a current control power transistor 12 during the start-up of the motor by providing a switch 25 and a resistor 23 connected to coils 13 to 15. At the start-up time, the resistor 23 consumes power from a power supply, and accordingly, causes a drop in the voltage supplied to the transistor 12 through the coils 13 to 15 and phase exchange transistors in a current drive circuit 6. After the start-up, the switch 25 is energized to bypass the resistor 23 so that a normal voltage from the power supply is supplied to the coils 13 to 15 and the transistor 12. The above energization of the switch 25 is carried out in response to a speed of the rotor.
This speed control circuit overcomes a part of the above problems, but since the above switching of the voltage-changeable supply circuit is essentially a single switching, the use of the speed control circuit is limited to only a small DC motor which has a short start-up time. In addition, the voltage-changeable supply circuit consisting of the switch and the resistor can not fully overcome the above problems.